The blades in the last few rows of a low pressure steam turbine are subject to erosion as a result of impact with moisture droplets which form in the steam in this area of the turbine. This erosion is most severe in the trailing edge portion of the airfoil near its tip. Since the ferrous alloys from which such blades are typically forged have high strength but less than optimum erosion resistance, in the past, the rate of such erosion has been retarded by brazing a thin strip of a wear resistant material, such as Stellite 6B.TM., onto the upper one-half to one-third of the trailing edge.
Unfortunately, brazing has several disadvantages. First, the metallurgical bond achieved by brazing is rarely 100% and bonds of only 80% must often be accepted. Second, the heat input associated with the brazing tempers the alloy from which the airfoil is forged, thereby lowering its mechanical strength and causing geometric distortion. Third, since microporosity is inherent in brazing, small pits are formed, thereby creating potential crack initiation sites.
It has long been known that good metallurgical bonding can be obtained by friction welding. Moreover, it has been suggested that friction welding be used to bond the airfoil portion of a turbine blade to its root portion or to a disc--see, U.S. Pat. Nos. 4,934,583 (Patsfall); 4,884,736 (Goddard) and 3,982,854 (Berry et al.). In addition, it has been suggested that friction surfacing, as such, can be applied to turbine blades--see, W. M. Thomas, "Solid Phase Cladding By Friction Surfacing", Welding for the Process Indiustries Paper 18 (1988). However, a method for utilizing a friction surfacing technique to apply a wear resistant material, such as Stellite 6B.TM., to the trailing edge of a steam turbine blade forged from the types of alloys typically used for such blades has not heretofore been developed.
As discussed in U.S. Pat. No. 4,079,491 (Richardson), the high temperature necessary to achieve the plasticity required for friction welding of wear resistant materials such as Stellite 6B.TM. to certain base alloys, such as stainless steel, imposes many practical difficulties owing the tendency for excessive temperature to be created in the base alloy. The problem of over-temperaturing the base alloy is especially important for steam turbine blades since the alloys from which they are formed are carefully heat-treated to obtain maximum strength and such strength can be lost by subsequent over-temperature.
Accordingly, it would be desirable to provide a workable method whereby a trailing edge could be formed on a steam turbine blade from a wear resistant material using a friction surfacing technique without generating excessive temperature in the blade.